In newly discovered oil fields, oil usually will be recovered by the oil flowing from a producing well under the naturally occurring pressure of the fluids present in the porous reservoir rocks. That naturally occurring pressure decreases as the fluids are removed. This phase of production, called primary production, recovers perhaps 5% to 20% of the oil present in the formation.
Secondary recovery methods (e.g., waterflooding) are used to recover more of the oil. In these methods, a fluid is injected into the reservoir to drive additional oil out of the rocks. Waterflooding has limitations. Since the water is immersible with oil, as the water displaces the oil the oil remaining in the reservoir reaches a limiting value known as "the residual oil saturation" and the oil stops flowing. There is a strong capillary action which tends to hold the oil in the interstices of the rocks. The amount of oil recovered by secondary techniques is usually from about 5% to 30% of the oil initially present.
In recent years, more attention has been directed to enhanced oil recovery or tertiary recovery methods. These tertiary recovery methods are used to recover the residual oil by overcoming the capillary forces which trap the oil during waterflooding. For example, it has been suggested to add surfactants to the flood to decrease the interfacial tension and thus allow oil droplets to move to producing wells.
Secondary or tertiary recovery of oil is also possible by the miscible fluid displacement process. A number of carbon dioxide floods have been tried in the United States. The carbon dioxide tends to dissolve in the oil, which swells with a consequent decrease in viscosity and improvement in the flow to producing wells. The carbon dioxide also extracts light hydrocarbons from the oil and this mixture of carbon dioxide and light hydrocarbons can, in some cases, reach a composition that will miscibly displace the oil.
This carbon dioxide-rich phase characteristically has a lower viscosity than the oil and tends to finger through the formation. Early carbon dioxide breakthrough is undesirable since reservoir sweep is reduced and expensive separation procedures are required to separate and recycle the carbon dioxide.
In U.S. Pat. No. 4,913,235, Harris et al. report a means of increasing viscosity for the carbon dioxide thirty-fold or more by using cosolvents, along with certain defined polymers having a solubility parameter of close to 6.85 (cal/cc).sup.1/2 [14.0 J.sup.1/2 /cm.sup.3/2 ] and having electron donor groups such as ether, silyl ether, and tertiary amine. Those defined polymers include polysiloxanes and polyvinylethers.